1. Field of the Invention
This invention directs itself to a method and a system for the bulk separation of single-walled tubular fullerenes based on their chiralities. More in particular, the present invention is directed to a method and a system of bulk separation which takes advantage of a phenomenon of preferential adsorption of single-walled tubular fullerenes at an energetically favored angular orientation of an axis of the lattice structure of a crystalline substrate with respect to an axis of a lattice structure of single-walled tubular fullerenes of a particular chiral angle or “chirality”. The method utilizes channels formed on the surface of the crystalline substrate that extend longitudinally thereacross at the energetically favored angular orientation. Still further, the present invention is directed to a system wherein at least one of the channeled crystalline substrate and a solution or suspension of the single-walled tubular fullerenes is displaced relative to the other for exposing at least a portion of the plurality of single-walled tubular fullerenes to the surface of the substrate exposed within the longitudinally extended channels.
2. Prior Art
The unique electrical and mechanical properties of tubular fullerenes and particularly carbon nanotubes for such applications as constructing nanoscale electronic circuitry and nanoscale mechanical devices has created a ever increasing demand for these structures. The electrical properties of carbon nanotubes can vary between metallic, highly conductive structures and those which are semiconducting as a function of their chirality, the angle at which the graphitic lattice spirals about the tubular contour of the nanotubes. The mechanical properties of carbon nanotubes also vary as a function of chirality. As a result of these characteristics, there is a demand for synthesizing the tubular fullerenes in a particular desired chirality required for an individual application. However, thus far the synthesis of carbon nanotubes produces a mixture of chiralities. Hence, there is a commercial need for a process to separate nanotubes of a desired chirality from all those produced.
In the earliest approach, a very tedious and inefficient mechanical separation process utilizing a nanoprobe (e.g., an atomic force microscope) was used to segregate nanotubes of a desired chirality from the mixture of chiralities produced. That method was a painstakingly slow process barely suitable to provide nanotubes for laboratory use.
An improvement over the mechanical separation technique is described in U.S. Pat. No. 6,669,918, which utilizes a template to which carbon nanotubes of a desired chirality are adsorbed thereto and subsequently removed therefrom. Using the property that carbon nanotubes of different chiralities adhere preferentially to a crystalline substrate with a lattice structure at different energetically favored angles with respect to a lattice axis of the substrate, a template is prepared using carbon nanotubes of the desired chirality which are deposited on a crystalline substrate and surrounded by a molecular layer of a material having a greater affinity for the substrate than the carbon nanotubes. The deposited nanotubes are removed to thereby provide openings in the molecular covering layer that are disposed at the angle that energetically favors adsorption of carbon nanotubes of the desired chirality therein. The template is then submerged in a solution or suspension of carbon nanotubes of mixed chiralities for sufficient time for random molecular motion to bring nanotubes of the desired chirality into proximity with the template openings. The adsorbed nanotubes are subsequently removed and the template re-submerged to separate further nanotubes of the desired chirality. This process had to rely on the mechanical separation technique to provide the nanotubes used to construct the template and repeated steps of a rather slow process for exposing the template opening to the nanotubes of the desired chirality.
A more direct method for interaction between the chiral surfaces of nanotubes of a desired chirality with the crystalline substrate followed, as described in U.S. Pat. No. 7,347,981. In this method, a fluid containing a plurality of nanotubes having a mixture of different chiralities is flowed across the surface of a crystalline substrate at the chiral angle that energetically favors adsorption of nanotubes having the desired chirality. The adsorbed nanotubes can then be removed from the substrate and the process repeated. A faster and far more efficient separation process over the template method, this directed flow method was not without complications. In order to separate the nanotubes of the desired chirality, the nanotubes in the flow had to have their longitudinal axes aligned with the flow so that they would be oriented at the energetically favored angle of the chirality selected for separation. While a number of techniques for aligning the axes of the nanotubes with the flow exist, it is still a requirement that adds complexity to the equipment and procedures that must be used to carry out this separation process.
Whereas the invention of the subject Patent Application utilizes the interaction between the chiral surfaces of nanotubes of a desired chirality with a crystalline substrate, the need to form a template and depend on random molecular motion, or axially align the nanotubes with a flow and to direct the flow at a critical angle is avoided. In the present invention the tubular fullerenes are brought into contact with a channeled crystalline substrate by relative displacement of one with respect to the other, without regard to the angular orientation of either. This simpler separation implementation is achieved by forming channels on the substrate surface that are oriented at the energetically favored angle for adsorption of the nanotubes of the desired chirality. Those randomly oriented nanotubes that are aligned with the channels will enter the channels and contact the substrate surface, with those of the desired chirality being adsorbed thereto. The nanotubes entering the channels that are of a chirality other than the desired chirality and any of the nanotubes not passing into a channel will not be held to the substrate surface. Despite the element of randomness associated with this method, the vast number of nanotubes present in any macroscopic volume of fluid, in either solution or suspension, provides sufficient efficiency of bulk chiral separation of nanotubes for a viable commercial process.